Scolytus intricatus
(European oak bark beetle)

S. intricatus is found throughout Europe, North Africa and the Near East (Iran and Turkey) (Wood and Bright, 1987).

Although adult flight is generally less than 100 m, the adults can easily be moved longer distances by the wind.

All life stages can be dispersed long distances via the movement of wood products including unprocessed logs and lumber, wooden crating, pallets and dunnage, containing strips of bark. S. intricatus was intercepted in dunnage at USA ports of entry 11 times between 1985 and 2000 (Haack, 2001).

North American oak forests are already under attack from several indigenous and introduced pests, including oak wilt (caused by the fungus, Ceratocystis fagacearum), a complex of biotic and abiotic factors leading to oak decline, defoliation by the gypsy moth, Lymantria dispar, and the recently discovered sudden oak death, caused by Phytophthora ramorum in California and southern Oregon. Increased mortality in oak forests due to the introduction and establishment of S. intricatus could lead to further stress on an already stressed ecosystem. Similar damage could occur if this insect appears in Asia.

S. intricatus invades and kills oaks that have been weakened by oak decline, drought or other stresses.

Breeding primarily occurs in oak, Quercus spp. (Duffy, 1953; Wood and Bright, 1992). In addition, Aesculus hippocastanum, Betula verrucosa, Carpinus betulus, Castanea sativa, Corylus sp., Fagus sylvatica, Ostrya carpinifolia, Populus sp., Salix sp., Sorbus sp. and Ulmus sp. have been listed as occasional hosts (Duffy, 1953; Schwenke, 1974; Lekander et al., 1977; Grüne, 1979; Schedl, 1981).

In laboratory tests, S. intricatus adults fed on the shoots of species of Aesculus, Alnus, Betula, Carpinus, Castanea, Eleagnus, Fagus, Juglans, Liquidambar, Morus, Prunus, Quercus, Salix and Ulmus (Doganlar and Schopf, 1984).

Life History and Habits

The genus Scolytus consists of approximately 55 species of bark beetles found in the forests of Asia, Europe and North America. Several species are considered to be important pests of either broadleaf or coniferous trees. Economically important species that are indigenous to North America include the fir engraver, Scolytus ventralis, the Douglas-fir engraver, Scolytus unispinosus and the hickory bark beetle, Scolytus quadrispinosus (Furniss and Carolin, 1977; Drooz, 1985). Scolytus morawitzi is a pest of larch in Asia. The smaller European elm bark beetle, Scolytus multistriatus, a vector of Dutch elm disease (caused by the fungi Ophiostoma ulmi and Ophiostoma novo-ulmi), was introduced into North America during the early part of the twentieth century and is now present over much of Canada and the USA.

The number of generations per year varies with location. For example, in England, S. intricatus completes one generation per year (Yates, 1984), whereas two generations per year are completed in southern Germany (Kamp, 1951).

S. intricatus typically overwinters in the late larval stages (Lekander et al., 1977; Yates, 1984) or occasionally in the pupal stage (Doganlar and Schopf, 1984). Overwintering and subsequent pupation usually takes place in the outer bark if the bark is over 4 mm thick, or in the outer sapwood if the bark is thin (Lekander et al., 1977; Yates, 1984). Pupation usually occurs in late spring or early summer and lasts for 1 to 2 weeks (Yates, 1984). In England, adult emergence usually spans 2 to 3 weeks and occurs from mid-May to late June, depending on local temperatures (Yates, 1984). S. intricatus has a 1:1 sex ratio at the time of adult emergence (Doganlar and Schopf, 1984; Yates, 1984).

Upon emergence, the adults fly to the crowns of trees, primarily oaks (Quercus spp.). There they undergo maturation feeding on the twigs, usually at the juncture of the current-year and 1-year-old growth (Munro, 1926; Lekander et al., 1977; Yates, 1984). In England, the period of shoot feeding is estimated to last for 2 to 3 weeks, assuming that this was the difference in time between peak adult emergence and peak attack on brood material (Yates, 1984). Doganlar and Schopf (1984) found that S. intricatus adults prefer to shoot-feed on Quercus but they also accepted species of Aesculus, Alnus, Betula, Carpinus, Castanea, Eleagnus, Fagus, Juglans, Liquidambar, Morus, Prunus, Salix and Ulmus in laboratory tests. In addition, Doganlar and Schopf (1984) noted that the shoot feeding did not occur on species of Acer, Celtis, Crataegus, Fraxinus, Liriodendron, Populus and Sambucus. Shoot feeding is not obligatory and newly emerged adults will reproduce if placed directly on freshly cut logs (Doganlar and Schopf, 1984; Yates 1984; Habermann and Schopf, 1987).

After shoot feeding, the adults seek breeding sites, which are usually the trunks and branches (> 5 cm in diameter) of weakened and dying oaks as well as recently fallen branches (Lekander et al., 1977; Gibbs, 1978; Yates, 1981, 1984). In the laboratory, S. intricatus successfully constructed egg galleries in branches that were only 1 to 1.5 cm in diameter (Doganlar and Schopf, 1984). S. intricatus has been reported to breed in oak trees stressed by drought (Gibbs, 1978; Yates, 1984; Novak, 1988; Eisenhauer, 1989), air pollution (Krol, 1982) and insect defoliation (Szontagh, 1984, 1985; Eisenhauer, 1989). In addition, S. intricatus will breed in branches that have been cut 12 to 18 months earlier (Yates, 1984). The pheromone system of S. intricatus has not been determined, but Yates (1984) suggested that the males might produce an aggregation pheromone. Mating may occur on twigs during maturation feeding (Doganlar and Schopf, 1984) or later during gallery formation (Yates, 1984). Adult flight may be limited to less than 100 m (Edelman and Malyseva, 1959). Depending on the location and the number of generations per year, adult activity usually occurs between May and September (Lekander et al., 1977; Yates, 1981, 1984; Doganlar and Schopf, 1984). There is little evidence that the re-emergence of adult parents occurs (Yates, 1984).

S. intricatus is monogamous. Either the males or females can initiate the construction of the egg galleries (Yates, 1984). The egg galleries are constructed perpendicular to the wood grain. The eggs are individually laid in niches that are constructed along both sides of the egg galleries. The average egg batch size was reported to range from 18 to 36 eggs per gallery in studies conducted in England (Yates, 1984). However, in Germany, average batch sizes of 36 to 83 eggs were reported (Doganlar and Schopf, 1984). The adult females typically lay eggs over a 2-week period and then die, usually within their egg galleries (Yates, 1984).

Egg hatch typically occurs from 10 to 14 days post egg lay (Yates, 1984). There are five larval instars (Yates, 1984) or possibly six (Doganlar, 1984). Pupation occurs in cells at the end of the larval galleries.

S. intricatus has been associated with oak decline throughout its geographic range (Szontagh, 1984; Kolk, 1985; Novak, 1988; Pavlik, 1994; Rossnev et al., 1994; Zach, 1994; Markovic and Stojanovic, 1996). In a few studies, S. intricatus adults were contaminated with fungal spores of Ophiostoma spp. at the time of emergence from brood trees (Edelman and Malyseva, 1959; Guseinov, 1984; Srutka, 1996). In other studies, S. intricatus was implicated as the principal vector of the various fungi associated with oak decline in Europe (Kryukova, 1976; Guseinov, 1981; Kolk, 1985; Eisenhauer, 1989; Rossnev et al., 1994). Many experts consider that S. intricatus would be an effective vector of the oak wilt fungus, Ceratocystis fagacearum if either the insect became established in North America or if the fungus became established in Europe (Gibbs, 1978, 1984; Gibbs and French, 1980; Doganlar et al., 1984; Doganlar and Schopf, 1984; Schopf et al., 1984; Yates, 1984).

Studies on the natural enemies associated with S. intricatus have been conducted in several locations (Yates, 1984; Capek, 1986; Vorst, 1994; Pavlik, 1994, 1999; Markovic and Stojanovic, 1996, 2003). Studies in Serbia indicated the presence of 20 parasitoid species in five families of Hymenoptera: Braconidae, Eurytomidae, Pteromalidae, Eupelmidae and Eulophidae. Ecphylus silesiacus had the greatest influence on the abundance of S. intricatus. Entendon ergias, Dendrosoter protuberans and Cheiropachus quadrum were also significant in reducing S. intricatus. E. ergias caused an average of 2.74% oak bark beetle parasitism, D. protuberans caused 2.63% and C. quadrum caused 1.63%. The average oak bark beetle parasitism for the study period from 1992 to 1996 was 14.49% (Markovic and Stojanovic, 1996).

A study in Slovakia indicated that the Great spotted woodpecker, Dendrocopos major destroyed between 47.7 and 72.9% of the S. intricatus brood in infested oak logs (Pavlik, 1999).

The bark surface should be inspected for boring dust. The presence of galleries and insect life stages should be looked for in the cambium and inner bark of unprocessed logs or dunnage, crating or pallets containing bark strips.

Most species of Scolytus are similar in appearance and difficult to identify to species. To ensure a positive identification, a taxonomist, with expertise in the family Scolytidae, should examine bark beetles believed to be a new introduction.

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

Cultural Control

Tree mortality, caused by S. intricatus, can be minimized through the maintenance of healthy, vigorous oak forests. This includes the rapid removal of trees or portions of trees damaged by the wind, ice or snow, or logging debris that could serve as breeding sites, thinning of dense oak forests and the removal of trees with symptoms of oak decline.

Biological Control

The opportunities for biological control of S. intricatus in its geographic range are limited because there is a diverse complex of natural enemies already attacking this insect (Yates, 1984; Capek, 1986; Pavlik, 1994; 1999; Markovic and Stojonovic, 1996, 2003). However, should this insect become established in a new location, some of the more commonly occurring species (e.g. Ecphylus silesiacus) would be candidate species for a classical biological control programme.

Chemical Control

The direct control of S. intricatus with chemicals is not considered to be a viable pest management tactic.

Pheromonal Control

Research is underway to study both the host- and insect-produced chemicals that influence host selection and mass attack by S. intricatus. Although some candidate compounds have been identified (Vrkocova et al., 2000, 2003), the use of these materials for monitoring or pest management of this insect has not yet been developed. Therefore, the use of pheromones for the monitoring or management of S. intricatus is not a viable tactic.

Field Monitoring

Due to the relationship between S. intricatus and European oak decline, monitoring for the two agents should be a co-ordinated effort. Crown symptoms, including thin and wilted foliage, branch dieback and tree mortality can be detected by a combination of aerial and ground surveys.

Integrated Pest Management

Few tactics are available to manage S. intricatus. Therefore, elements of an integrated pest management programme must focus on measures designed to maintain the health and vigour of oak forests. This could be coupled with the timely detection and removal of material that could serve as breeding sites for this insect (e.g. logging debris, damaged or broken trees or trees with symptoms of oak decline).